Prior to setting forth a short discussion of the related art, it may be helpful to set forth definitions of certain terms that will be used hereinafter.
The term “MIMO” as used herein, is defined as the use of multiple antennas at both the transmitter and receiver to improve communication performance. MIMO offers significant increases in data throughput and link range without additional bandwidth or increased transmit power. It achieves this goal by spreading the transmit power over the antennas to achieve spatial multiplexing that improves the spectral efficiency (more bits per second per Hz of bandwidth) or to achieve a diversity gain that improves the link reliability (reduced fading), or increased antenna directivity.
The term “TDD” (Time Division Duplex) as used herein, is defined as the use of the same or a single channel (e.g., the same or overlapping frequency spectrum) for both downlink and uplink transmissions. For example, TDD communication, e.g., between a mobile device and a base station, may periodically alternate between uplink transmissions (e.g. from the mobile device to a base station for a predetermined uplink interval or period, such as, 5 milliseconds (ms)) and downlink transmissions (e.g. from a base station to a mobile device or UE for a predetermined downlink period, such as, 5 ms). The base station typically coordinates the alternating timing between uplink (UL) and downlink (DL) transmissions.
The term “beamforming” sometimes referred to as “spatial filtering” as used herein, is a signal processing technique used in antenna arrays for directional signal transmission or reception. This is achieved by combining elements in the array in such a way that signals at particular angles experience constructive interference while others experience destructive interference. Beamforming can be used at both the transmitting and receiving ends in order to achieve spatial selectivity.
The term “beamformer” as used herein refers to RF circuitry that implements beamforming and usually includes a combiner and may further include switches, controllable phase shifters, and in some cases amplifiers and/or attenuators.
A transmitter in a MIMO system requires channel state information (CSI) so that a set of precoded weights can be set to the multiple data streams of the transmitter, in order to exploit the channels for the multiple spatial channel transmission. Typically, the receiver can feed the CSI or even the preferred pre-coding matrix (index) back to the transmitter. These feedbacks can consume some available bandwidth of the transmission in the opposite direction and reduce the data throughput. If transmission in both directions operate in the same spectrum, like in the case of time division duplex (TDD) systems, the channels through the air are reversible and the channel information can be estimated by the receive device and then applied to the transmit device.
However, a complete transmission channel should be from the transmitter baseband to the receiver baseband, which includes various components inside the transmitter (e.g., DAC, up converter, power amplifier, filter) and receiver (e.g., duplexer, LNA, down converter, filter, ADC). The transmission path and receive path may thus experience very different gain/loss and delays behavior, due to the different components used in both paths. Channel reciprocity without considering the different delay and gain/loss factors between the transmit/receive paths are therefore not valid and may not be accurate enough for the use by devices in TDD MIMO systems. These parameters can also be factory calibrated. One important element that jeopardizes reciprocity is antennas, which project slightly different radiation patterns at Up and Down Links, due to differences in the Voltage Standing Wave Ratio (VSWR) in both directions. Antenna VSWR cannot be practically calibrated in the factory, due to the cost of such procedures.
In beamforming applications there are benefits in using uplink signals' channel estimation for downlink beamforming, i.e. for establishment of calculated reciprocity A method and apparatus for self-calibration of the transmitter and receiver paths is disclosed herein (between base band and transmit/receive antennas) of a MIMO device to obtain the precise channel reciprocity information necessary for improving the (TDD) MIMO systems.